1. Field of the Invention
The present invention relates to semiconductor memory devices and systems using the semiconductor memory devices. More particularly, the present invention relates to a semiconductor memory device, system, and a method for using a semiconductor memory device having different data rates for a memory read and a memory write operation.
2. Description of the Related Art
In a system using a memory device, the memory device receives a write data signal transferred from a memory controller in a write operation. Alternatively, the memory controller receives a read data signal outputted from the memory device in a read operation.
The memory controller needs a sampling signal synchronized with the read data signal to perform a data sampling operation for the read data outputted from the memory device in the read operation. Hence, generally, the memory device outputs the read data signal synchronized with a system clock, and the memory controller performs the data sampling operation by using the system clock as the sampling signal. Alternatively, the memory device in the read operation generates a separate strobe signal and outputs the strobe signal together with the read data signal, and the memory controller performs the data sampling operation using the strobe signal as the sampling signal.
In a case where the memory device generates the separate strobe signal in a read operation, the memory device can be configured such that the read data signal and the strobe signal are either synchronized with the system clock or set to have a predetermined phase difference to the system clock. To synchronize the read data signal and the strobe signal with the system clock, a phase synchronization device such as delay locked loop (DLL) or phase locked loop (PLL) is typically used. An exemplary memory device having such a phase synchronization device is a double data rate (DDR) synchronous dynamic random access memory (SDRAM).
However, even with such synchronization, phase differences during the read data operation can still occur due to different path lengths in the coupling between the memory controller and the memory device and in the buffering stages in the two devices. Similar loss of synchronization can occur during a write data operation.
Explaining in greater detail, one strobe signal (or system clock) per eight-bits of data is generally transferred to the memory device in a write operation, a strobe signal (or system clock) input buffer receives and buffers the strobe signal (or system clock), and eight data input buffers receive and buffer the eight-bits of data. Next, eight data sampling circuits simultaneously perform a data sampling operation for the buffered eight-bits of data in response to the buffered strobe signal (or the buffered system clock) simultaneously. The output of each data input buffer is connected to the input of each data sampling circuit, respectively, and the output of the strobe signal (or system clock) input buffer is connected to the control inputs of the eight data sampling circuits. Further, the loading of the output of the strobe signal (or system clock) input buffer may be much larger than that of each data input buffer, thereby changing the time that the strobe signal (or system clock) is delayed
Due to the time difference of the delays, even if the write data and the strobe signal (or system clock) inputted to the memory device are externally synchronized with each other, loss of synchronization between the buffered write data by the data input buffer and the buffered strobe signal (or the buffered system clock) by the strobe signal (or system clock) input buffer can occur, and data set-up and hold characteristics may be significantly degraded. This phenomenon gets worse with increased data transfer speeds, that is, when bit-time is reduced.
Hence, the memory device used for high speed input data sampling operations typically uses DLL or PLL to solve the above synchronization problems by generating a compensated internal strobe signal (or internal clock) to perform the data sampling operations. Disadvantageously, incorporation of synchronization devices such as DLL or PLL in the memory device, contributes to design complexity, increased chip area, and yield deterioration.
It is therefore a feature of an embodiment of the present invention to provide a semiconductor memory device that can perform a data sampling operation safely in a write operation without a phase synchronization device such as DLL or PLL.
It is another feature of an embodiment of the present invention to provide a method for controlling data input/output of a semiconductor memory device that can perform a data sampling operation safely in a write operation without the phase synchronization device such as DLL or PLL.
It is another feature of an embodiment of the present invention to provide a system in which a semiconductor memory device can perform a data sampling operation safely in write operation without a phase synchronization device such as DLL or PLL.
According to an aspect of an embodiment of the present invention, a semiconductor memory device is provided which includes an output section generating a first strobe signal synchronized with a read data signal in a read operation, and outputting the read data signal at a rising edge and a falling edge of the first strobe signal, and an input section receiving a write data signal at either a rising edge or a falling edge of a second strobe signal generated in a memory controller or a system clock in a write operation.
According to a preferred embodiment of the present invention, the first strobe signal is generated through the same path as the output path of the read data.
The memory controller performs a data sampling operation on the read data signal using the first strobe signal in the read operation. Also, the memory controller transfers the write data to the semiconductor memory device in synchronization with the system clock or the second strobe signal in the write operation.
According to another aspect of an embodiment of the present invention, a method for controlling data input/output of a semiconductor memory device includes generating a first strobe signal synchronized with a read data signal in a read operation, and outputting the read data signal at both a rising edge and a falling edge of the first strobe signal, and receiving a write data signal at either a rising edge or a falling edge of a second strobe signal generated in a memory controller or a system clock in a write operation.
According to another feature of an embodiment of the present invention, a system includes a semiconductor memory device operating in response to a system clock, and a memory controller operating in response to the system clock, controlling the semiconductor memory device, and exchanging data with the semiconductor memory device, wherein a data rate of a write data signal transferred to the semiconductor memory device from the memory controller in a write operation is different from a data rate of a read data signal transferred to the memory controller from the semiconductor memory device in a read operation.
According to a preferred embodiment of the present invention, the data rate of the write data signal is half of the data rate of the read data signal.
According to a preferred embodiment of the present invention, the semiconductor memory device includes an output section generating a first strobe signal synchronized with the read data signal in the read operation and outputting the read data signal at a rising edge and a falling edge of the first strobe signal respectively, and an input section receiving the write data signal at either a rising edge or a falling edge of a second strobe signal generated in the memory controller or the system clock in the write operation.
According to a preferred embodiment of the present invention, the first strobe signal is generated through the same path as an output path of the read data signal.
The memory controller performs a data sampling operation for the read data signal using the first strobe signal in the read operation. Also, the memory controller transfers the write data signal to the semiconductor memory device in synchronization with a system clock signal or the second strobe signal in the write operation.
These and other features of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.